1. Field of Invention
The present invention relates to mapping the open natural fractures in the petroleum reservoirs, more particularly identifying their location and their orientation in the existing oil fields.
2. Description of the Prior Art
In most of the carbonate and certain sandstone reservoirs natural fractures are encountered that are open and control the directional permeability and the effective flow pathways for the hydrocarbons. Mapping these fractures and their orientation is the key to the economic recovery of hydrocarbons from these reservoirs. At present, natural fracture characterization is of increasing importance, since the industry is venturing into increasing their producible reserves from the existing fields that are showing production decline.
Natural fractures in the subsurface rocks are usually vertical and are mostly found in the formations that have gone through structural deformation or have experienced regional stresses. These fractures commonly terminate at lithologic discontinuities within the reservoir formations. These fractures can be closely or widely spaced and irregularly distributed. Quite often, swarms of fractures are encountered with unfractured intervals in between. Economic hydrocarbon production from the fractured reservoirs requires an optimal access of the wellbore to the open fractures. This makes it extremely important that an accurate map of the open fracture system should be available prior to any field infill and development program.
In many cases, subsurface fractures are difficult or impossible to map adequately by using currently available technologies. Physical measurements through cores and well logs are limited to the vicinity of the wells drilled in the reservoir. The density of sampling the reservoir rock using cores and well logs quite often is not sufficient to provide any useful information regarding the orientation and the location of the fractures. This is due to two main characteristics of the majority of the wells that are drilled:
1) Both the wells and the fractures are generally vertical and parallel to each other; and
2) The wellbore is smaller than the fracture spacing between the larger fractures.
Horizontal drillingxe2x80x94where the cost of drilling a well is highxe2x80x94has to be designed to take the full advantage of the natural fractures that are open by mapping their location and their orientation. Since a single horizontal well is limited in producing from a few layers of the reservoir, it is important to identify the part of the reservoir from which the production can be optimized prior to drilling the well. This requires that the specific fractured beds should be identified prior to any drilling commitments.
This invention uses the nonlinearity of the seismic waves and its measurements to characterize the fractures as the waves propagate through the fractured rocks. Two seismic signals are used. One is a high-frequency seismic signal (the xe2x80x98carrierxe2x80x99 wave) transmitted from a wellbore, which penetrates the reservoir and travels through the fractured rock and is recorded by the receivers in another well. The other is a lower-frequency seismic signal (the xe2x80x98modulationxe2x80x99 wave) that is transmitted from the surface using a movable source like a surface seismic vibrator. In both cases, sinusoidal seismic signals of pre-selected frequencies are used. The lower-frequency source is located at predeterminned locations on the surface to modulate the open subsurface fractures with its compression and rarefaction alternate cycles.
The transmission characteristics of the fractured rock are measured (1) as the open fractures tend to close or are squeezed on a compression cycle; and (2) as the fractures tend to open on a rarefaction cycle of the low-frequency xe2x80x98modulationxe2x80x99 seismic signal.
The nonlinearity measurement can be made by analyzing the amplitude and the harmonics of the high-frequency xe2x80x98carrierxe2x80x99 wave during compression and by analyzing the rarefaction cycles of the low-frequency xe2x80x98modulationxe2x80x99 wave. During the compression cycle the stress across the fractures is increased; this tends to partially close the fractures thus reducing their nonlinearity effect and increasing the amplitude of the xe2x80x98carrierxe2x80x99 wave transmission. During the rarefaction cycle the stress across the fractures is decreased, thus tending to open the fractures and making their physical characteristics more nonlinear and reducing the amplitude of the high-frequency signal. During the compression and rarefaction cycles of the xe2x80x98modulationxe2x80x99 wave, the differences caused in the xe2x80x98carrierxe2x80x99 seismic wave due to nonlinearity are measured by the relative amplitude of the xe2x80x98carrierxe2x80x99 wave and its harmonics. This difference is maximum when the surface source that generates the lower-frequency xe2x80x98modulationxe2x80x99 wave is located at or near right angles to the open fractures.
This difference, mentioned above, will be zero when the surface source is located parallel to the fractures or directly above them, since the xe2x80x98modulationxe2x80x99 signal will have no squeezing effect on the width of the open fractures.
By moving the surface source to different locations on the surface and making measurements on the xe2x80x98carrierxe2x80x99 wave, the orientation of the fractures can be determined.
Since practically all the subsurface fractures are vertical, the fracture width of the open fractures is not modulated when the surface source is directly above them or aligned with the same angle as the fractures. The xe2x80x98modulationxe2x80x99 is maximum when the surface source is (1) at or near right angles to the fractures and (2) at a distant offset, so that the xe2x80x98modulationxe2x80x99 seismic signal is arriving at the fracture at a wide angle.
Once the fracture orientation is established, its location can be determined by moving the surface source in a straight line at right angles to the fractures until the differences in the amplitude and the nonlinearity between compression and rarefaction cycles becomes zero. At that point, the fractures are located directly below or along the plane of transmission of the surface xe2x80x98modulationxe2x80x99 source.
Briefly, the present invention provides a new and an accurate seismic method of mapping the orientation and location of the open natural fractures that are common in the hydrocarbon reservoirs. This invention creates a change in a controlled manner and uses the measurements of that change to characterize the fractured rock.
Two discrete seismic frequency signals are used. One is a high-frequency signal referred to in the description as a xe2x80x98carrierxe2x80x99 signal which is in the order of one hundred times higher frequency than the other lower-frequency signal, which is termed as a xe2x80x98modulationxe2x80x99 signal. The xe2x80x98carrierxe2x80x99 signal is transmitted using a seismic source located in a wellbore with multiple receivers located in an adjacent wellbore. More than one wellbore can be used for receivers to listen simultaneously and each wellbore can have multiple receivers, each receiver with its own independent output. The borehole source can be moved up and down in the wellbore to cover different formations in the reservoir that may be fractured.
The lower-frequency xe2x80x98modulationxe2x80x99 signal source is located on the surface and can easily be deployed in any geometric pattern that is considered necessary to map the location and orientation of the fractures. Normally the surface sources can transmit higher energy seismic signals compared to the downhole transmitter; their signal strength can be in the order of one hundred times larger than the downhole source. Additionally, the lower frequencies are less attenuated as they travel through the earth. So the amplitude level of the xe2x80x98modulationxe2x80x99 signal available at the subsurface fractures can be very much larger than the high-frequency xe2x80x98carrierxe2x80x99 signal generated by the downhole source.
Experiments in rocks reveal a large nonlinear elastic wave response, far greater than that of gases, liquids, and most other solids. The large response is attributed to structural discontinuities, such as fractures, in the rocks (P. A. Johnson and K. R. McCall, Los Alamos National Laboratory, Los Alamos, N. Mex.). This nonlinear wave behavior implies that as the seismic wave propagates through the rock there is a local increase in the density and modulus during compression and a local decrease in density and modulus during rarefaction. The wave form begins to change shape and harmonics are generated. This effect is cumulative.
A large amplitude, low-frequency xe2x80x98modulationxe2x80x99 signal squeezes the open fractures during the compression cycle and opens them during the rarefaction cycle. The transmission of the xe2x80x98carrierxe2x80x99 wave through the swarm of fractures is affected by the compression and rarefaction cycles of the xe2x80x98modulationxe2x80x99 wave. Since the frequency of the xe2x80x98carrierxe2x80x99 wave is higher by roughly a factor of 100 and the signal strength of the low-frequency xe2x80x98modulationxe2x80x99 wave is considerably stronger, the interaction of the two waves can be measured. This interaction of the two waves, as they are transmitted through multiple open fractures, produces distortion and harmonics of the xe2x80x98carrierxe2x80x99 frequency. The harmonic content and the amplitude of the xe2x80x98carrierxe2x80x99 wave changes during the compression and the rarefaction periods of the xe2x80x98modulationxe2x80x99 wave. The effect, as the seismic signal travels through multiple fractures of a swarm, is cumulative.
The difference in the amplitude and the harmonic distortion of the xe2x80x98carrierxe2x80x99 wave between compression and rarefaction cycles of the xe2x80x98modulationxe2x80x99 wave is indicative of the angle at which the xe2x80x98modulationxe2x80x99 wave is intersecting the fractures. The maximum difference will occur when the xe2x80x98modulationxe2x80x99 wave is at right angles to the fractures and zero or near zero difference will occur when the xe2x80x98modulationxe2x80x99 signal is arriving from directly above or at a direction that is parallel to the fractures.
The measurements and the analyses of the xe2x80x98carrierxe2x80x99 wave signals recorded by the receiver array and transmitted by the downhole source are made for all the downhole source locations. The output from each source location is recorded by the receiver array, which may be 100 independent receiver signals, sampling the receiver well every 5 or 10 feet. The receiver array is long enough to provide the vertical coverage for the formations of interest. Each data set, when completed, will be a matrix xe2x80x98nxe2x80x99 number of downhole source locations 5 feet or 10 feet apart recorded in all the receivers in the receiver array, which may be 100 independent receiver signals. The xe2x80x98nxe2x80x99 number of downhole source locations will be sufficient to vertically sample the formations that are part of the reservoir suspected to have open fractures.
The entire recording procedure is repeated for multiple surface source locations, deployed in a geometric pattern. To describe it, a simple pattern is used to explain the concept. In real life, any other suitable pattern can be designed; multiple receiver wells can be used to record the data using a single source well to provide aerial coverage of the open fractures in the reservoir. The knowledge for such a design is known in the current art.